AuthorTopic: Another U47/NG conundrum (Read 3036 times)

Okay so back at getting my U47 voltages working right. At first I thought my NG PSU might be the problem but what I found out is that if I load the PSU with a different U47 (and keeping the NG resistor values stock) I reach 104VDC consistently. But when I put the U47 that I'm working on on the same PSU, my voltages rise 5 or 6 VDC. So this leads me to believe that some values might be amiss in the mic itself and not the NG PSU. Now I can easily change the PSU values (non stock) so that I can get a proper 104 VDC, but I'm not sure if this is the correct thing to do.

Advice?

Thank-you.

EDIT: I almost forgot. I also measured f1 & f2 from the contact on the vf14 connector and get 37.75 volts at f1 and 1.2 volts at f2 (according to the schematic this should be reversed where f2 gets the 36 volt reading). Anyhow, these readings are with the PSU measuring 110 volts.

A little more testing: If I adjust the PSU down to 107V loaded, I get a measurement at f1 precisely at 36v and f2 1.175v (again I'm not sure why f1 and f2 are reversed according to schematic?). And lastly (if I took the measurement at the right spot) it seems like the capsule is only being supplied 58v.

..and the question is...?I assume you would like clarification on two issues:

1. Why do different mics draw different amounts of current from the same power supply? Because the tube current draw differs slightly from tube to tube, as the filament wire resistance drops with wear of the tube, and the tolerance range of components in the mic adds another volt or so in difference.

Bottom line: you always need to custom-adjust the supply voltage for the mic you use with it, and monitor or adjust it once a year or so to keep it such that the prescribed voltages of 36V heater, at ca. 105-107 B+ remain.

2. Yes, the designations for F1 and F2 on most Neumann U47 schematics are reversed from what you will find in the actual circuit.

Yes, if it's truly 58VDC, it is marginally fine. But if you follow what I wrote in my previous post, adjusting the power supply correctly and permanently to the mic that will be used with it is the better way to go.

I know, it's a pain in the butt to substitute bridging resistors in NG or NG1 power supplies rather than the convenience of a trim pot, but in your case it's quite easy: in case you have the NG1 supply, leave all four 1K resistors (or whatever is in there now) alone, and add a ca. 200Ω 10W resistor before the first resistor, right after the rectifier.

In case of the NG supply, change out the single bridging resistor mounted between the two chokes to a slightly higher value, again 10W rating. It's not that expensive to get a stash resistors with varying resistances.

Big disclaimer and warning: Do not attempt any of this if you are not well-versed in high voltage eletrics! It can be deadly!